Desalination 289 (2012) 66–71 Contents lists available at SciVerse ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal Removal of high-concentration phosphate by calcite: Effect of sulfate and pH Yun Liu a,⁎, Xia Sheng b, Yuanhua Dong a,⁎, Yijie Ma a a Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, 210008 Nanjing, China b Mineral construction branch, Yanzhou Mining Group, Donghua Construction Co., Ltd. Yanzhou Mining Group, the building of the Tung Wah Group of Geology and Mineral Company, Zoucheng, 273500 China article info abstract Article history: Sulfate is a common anion in wastewater, and the effect of sulfate on high-concentration phosphate removal Received 13 October 2011 by calcite has not been fully investigated. We studied high-concentration phosphate removal as a function of Received in revised form 13 January 2012 pH in both the presence and absence of sulfate. Solution pH greatly affected phosphate-removal mechanism Accepted 14 January 2012 via calcite and phosphate-calcite reaction products. Phosphate removal was favorable in both acidic and Available online 9 February 2012 strong basic solutions; solutions with an initial pH between 8.0 and 11.0 were significantly less effective. The effect of sulfate on phosphate removal depended on solution pH. In the acidic solution, phosphate remov- Keywords: Calcite al was enhanced by a low concentration of sulfate but inhibited by a high concentration of sulfate (i.e., a Phosphate sulfate-to-phosphate ratio greater than 6.0). Although sulfate can promote calcite to dissolve, high concen- Sulfate trations of sulfate compete with phosphate-forming calcium sulfate first as precursor, and calcium sulfate Kinetics eventually converts into Hydroxyapatite (HAp). In the strong base media, phosphate removal increased with increasing sulfate concentration. This increase in phosphate removal was attributed to the increase of solubility of calcite in the presence of sulfate. The process of phosphate removal fit a four-parameter logistic function model regardless of the presence of sulfate, but it did not fit traditional kinetic models. Published by Elsevier B.V. 1. Introduction adsorption isotherms, probably due to monolayer adsorption. Kuo et al. reported that about 80% of phosphate anions were adsorbed within Phosphorus has long been recognized as the most critical nutrient 10 s, possibly because phosphate ions replaced adsorbed water mole- that limits water productivity. The wide use of phosphorus in indus- cules, bicarbonate ions, or hydroxyl ions from the calcite surfaces [13]. trial products such as fertilizers, detergents, pesticides, and water Multilayer adsorption seems to also occur at specific sites on calcite sur- softeners, etc., produces large amounts of phosphorus-bearing waste- faces. Adsorption of phosphate can take place on a limited number of water. Especially in the manufacture of paints, the phosphate concen- sites, and as the site coverage increases, lateral interaction occurs be- tration of wastewater from the degreasing, phosphating and painting tween the adsorbed phosphate and free ions. These clusters can serve departments can reach thousands of mg/L coexisting with numerous as heteronuclei from which spontaneous crystal growth can occur 2+ 2+ 2− 2− cations and anions (such as Mg ,Ca ,CO3 ,SO4 , etc.) in a wide [10]. At high phosphate concentrations, the reaction between phos- pH range. The sulfate concentration sometimes approaches its satura- phate and calcite surfaces starts with small amounts of phosphate ad- tion level. Before wastewater is discharged into water bodies, remov- sorption, followed by precipitation as dicalcium, brushite, octacalcium ing phosphate is usually obligatory [1]. Many technologies have been phosphate, and HAP [4,11]. As the concentration of phosphate in- applied to remove phosphate from wastewater, including precipita- creases, debris appears on the surface in the form of small lumps tion of phosphate by metal salts, cultivation of microorganisms in about 1 μm in diameter and coral-like porous hemispherical growths wastewater, constructed wetlands, and enhanced biological phospho- on the faces and edges of the crystals [11]. pH plays an important role rus removal (EBPR) [1]. in the removal of phosphate. In solutions with low phosphate concen- Research has proven that phosphate can be effectively removed by trations, high pH favors the removal process of phosphate species. At calcite [2–8]. The reaction mechanisms of phosphate with calcite pH levels between 7.0 and 8.2, adsorption seems to result from electro- further reveal that phosphate can be either adsorbed by calcite at a static and chemical interactions [3]. A study examining solutions with low concentration, typically b20 μmol/L [4,9], or precipitated at a high high phosphate concentrations and pH in ranging from 4 to 12 showed concentration [4–11]. Detailed studies have been conducted on the ad- that the highest phosphate removal efficiency was obtained in the pH sorption of phosphate by calcite at low phosphate concentrations [3,12]. range of 6 to 7, and the efficiency decreased with increasing pH [4]. The adsorption of phosphate by calcite can be described by Langmuir 2+ 2+ 2− 2− Many cations and anions (such as Mg ,Ca ,CO3 ,SO4 , etc.) in industrial wastewater may also affect phosphate removal by cal- 2+ 2+ ⁎ Corresponding authors. Tel.: +86 25 86881317; fax: +86 25 86881000. cite. The presence of Mg and Ca can increase the adsorption of E-mail addresses: [email protected] (Y. Dong), [email protected] (Y. Liu). phosphate by calcite because these cations can form CO3\Mg\PO4 0011-9164/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.desal.2012.01.011 Y. Liu et al. / Desalination 289 (2012) 66–71 67 and CO3\Ca\PO4 bonds at the carbonate sites on the calcite surface ray diffractometer (Rigaku Corp., Tokyo, Japan) with CuKa radiation 2− [12]. Adding SO4 to the solution may lower the activity of these cat- (40 kV, 40 mA) and a Ni filter, scanned from 3.0° to 50.0° with a ions due to the formation of sulfate ion pairs (MgSO4 and CaSO4), and scan speed of 4.0°/min. thus reduce phosphorus adsorption. The adsorption of phosphate on calcite is almost independent of salinity, but is strongly influenced 3. Results and discussion by bicarbonate concentration [12]. Fluoride can decrease phosphate removal regardless of pH because fluoride competes for calcium to 3.1. Effect of pH form CaF2 [4,14,15]. There is currently no data available on the effect of sulfate on Fig. 1 shows the relation between the pH and phosphate removal phosphate removal and the kinetics of phosphate removal by calcite at 48 h. When initial pH was increased from 3.0 to 5.0 (the final pH at high phosphate concentrations. The objective of this study was to from 6.6 to 7.7), the phosphate removal increased with increasing investigate the effects of sulfate on high-concentration phosphate re- pH. Phosphate removal decreased with the increase of initial pH moval by calcite at different pHs. from 5.0 to 10.0 (final pH from 7.7 to 9.2). Less efficient phosphate re- moval was observed at an initial pH range from 8.0 to 11.0, and the 2. Experiment methods removal efficiency was then increased with the increase of pH from 10.0 to 13.0 (final pH from 9.2 to 13.0). 2.1. Materials and chemicals The effect of pH on phosphate removal can be attributed to changes in the surface sites or speciation of phosphorus in the solu- A natural calcite sample was mined from Zhenjiang County tion as well as the dissolution of calcite [4,12]. The effect of pH on (China), and the content of calcium carbonate was 98.4% wt. The cal- phosphate removal is also related to the degree of supersaturation cite sample was manually ground, and particles between 80 and 100 and the type of precipitates formed [4]. As we know, when phosphate mesh were selected. A solution containing 1000 mg P/L phosphate (P or calcite exists in water, the following chemical species will be stock solution) was prepared by dissolving pure KH2PO4 in distilled formed by the following reactions at 25 °C [3,17–20]. water. The pH of the initial phosphate solution was then adjusted to 3− þ ↔ 2− þ − ¼ : ð Þ a desired value using KOH or HCl solutions. PO4 H2O HPO4 OH pK 1 68 1 2.2. Influence of pH on removal of phosphate 2− þ ↔ − þ − ¼ : ð Þ HPO4 H2O H2PO4 OH pK 6 79 2 The effect of pH on phosphate removal was evaluated at different − þ ↔ þ − ¼ : ð Þ H2PO4 H2O H3PO4 OH pK 11 67 3 initial pH values (3.0–13.0) at room temperature (25 °C) for 48 h. Exact- ly 1.00 g calcite and 50 mL solution with a P concentration of 1000 mg/L ↔ þ þ − ¼ ð Þ H2O H OH pK 14 4 (pH pre-adjusted to the desired value) were mixed in a 150 mL flask. The mixture was stirred at 150 rpm at 25 °C in a thermostatic shaker. − : CaCO ðÞs ↔CaCO ðÞaq K1 ¼ 10 5 09 ð5Þ After 48 h, the suspension was filtered through a 0.45 μm cellulose ace- 3 3 tate membrane and the filtrate was analyzed for residual phosphate ðÞ↔ 2þ þ 2− ¼ −3:25 ð Þ concentration. The analysis of phosphate (as phosphorous) was done CaCO3 aq Ca CO3 K2 10 6 following the molybdenum-blue ascorbic acid method with a UV–vis 2− þ ↔ 3− þ − ¼ −3:67 ð Þ spectrophotometer (UV/VIS 1600) (Beijing Ruili Analyzing Instrument CO3 H2O HCO OH K3 10 7 Factory, Beijing, China) [16]. − þ ↔ þ − ¼ −7:65 ð Þ HCO3 H2O H2CO3 OH K4 10 8 2.3.